Chapter 3 Using Classes and Objects pptx

Chapter 3
Using Classes and
Objects
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Using Classes and Objects
•
We can create more interesting programs using
predefined classes and related objects
•
Chapter 3 focuses on:

object creation and object references

the String class and its methods

the Java standard class library

the Random and Math classes

formatting output

enumerated types

wrapper classes

graphical components and containers

labels and images
© 2004 Pearson Addison-Wesley. All rights reserved 3-3
Outline
Creating Objects

The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2004 Pearson Addison-Wesley. All rights reserved 3-4
Creating Objects
•
A variable holds either a primitive type or a
reference to an object
•
A class name can be used as a type to declare an
object reference variable
String title;
•
No object is created with this declaration
•
An object reference variable holds the address of
an object
•
The object itself must be created separately
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Creating Objects
•
Generally, we use the new operator to create an
object
title = new String ("Java Software Solutions");
This calls the String constructor, which is

a special method that sets up the object
•
Creating an object is called instantiation
•
An object is an instance of a particular class
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Invoking Methods
•
We've seen that once an object has been
instantiated, we can use the dot operator to invoke
its methods
count = title.length()
•
A method may return a value, which can be used
in an assignment or expression
•
A method invocation can be thought of as asking
an object to perform a service
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References
•
Note that a primitive variable contains the value
itself, but an object variable contains the address
of the object
•
An object reference can be thought of as a pointer
to the location of the object
•
Rather than dealing with arbitrary addresses, we
often depict a reference graphically

"Steve Jobs"
name1
num1
38
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Assignment Revisited
•
The act of assignment takes a copy of a value and
stores it in a variable
•
For primitive types:
num1
38
num2
96
Before:
num2 = num1;
num1
38
num2
38
After:
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Reference Assignment
•
For object references, assignment copies the
address:
name2 = name1;
name1
name2

Before:
"Steve Jobs"
"Steve Wozniak"
name1
name2
After:
"Steve Jobs"
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Aliases
•
Two or more references that refer to the same
object are called aliases of each other
•
That creates an interesting situation: one object
can be accessed using multiple reference
variables
•
Aliases can be useful, but should be managed
carefully
•
Changing an object through one reference
changes it for all of its aliases, because there is
really only one object
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Garbage Collection
•
When an object no longer has any valid references
to it, it can no longer be accessed by the program
•
The object is useless, and therefore is called

garbage
•
Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use
•
In other languages, the programmer is responsible
for performing garbage collection
© 2004 Pearson Addison-Wesley. All rights reserved 3-12
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2004 Pearson Addison-Wesley. All rights reserved 3-13
The String Class
•
Because strings are so common, we don't have to
use the new operator to create a String object
title = "Java Software Solutions";
•
This is special syntax that works only for strings
•
Each string literal (enclosed in double quotes)
represents a String object
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String Methods
•
Once a String object has been created, neither its
value nor its length can be changed
•
Thus we say that an object of the String class is
immutable
•
However, several methods of the String class
return new String objects that are modified
versions of the original
•
See the list of String methods on page 119 and in
Appendix M
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String Indexes
•
It is occasionally helpful to refer to a particular
character within a string
•
This can be done by specifying the character's
numeric index
•
The indexes begin at zero in each string
•
In the string "Hello", the character 'H' is at index
0 and the 'o' is at index 4
•
See StringMutation.java (page 120)
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Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2004 Pearson Addison-Wesley. All rights reserved 3-17
Class Libraries
•
A class library is a collection of classes that we
can use when developing programs
•
The Java standard class library is part of any Java
development environment
•
Its classes are not part of the Java language per
se, but we rely on them heavily
•
Various classes we've already used (System ,
Scanner, String) are part of the Java standard
class library
•
Other class libraries can be obtained through third
party vendors, or you can create them yourself
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Packages
•

The classes of the Java standard class library are
organized into packages
•
Some of the packages in the standard class library
are:
Package
java.lang
java.applet
java.awt
javax.swing
java.net
java.util
javax.xml.parsers
Purpose
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication
Utilities
XML document processing
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The import Declaration
•
When you want to use a class from a package, you
could use its fully qualified name
java.util.Scanner
•
Or you can import the class, and then use just the
class name

import java.util.Scanner;
•
To import all classes in a particular package, you
can use the * wildcard character
import java.util.*;
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The import Declaration
•
All classes of the java.lang package are
imported automatically into all programs
•
It's as if all programs contain the following line:
import java.lang.*;
•
That's why we didn't have to import the System or
String classes explicitly in earlier programs
•
The Scanner class, on the other hand, is part of
the java.util package, and therefore must be
imported
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The Random Class
•
The Random class is part of the java.util
package
•
It provides methods that generate pseudorandom
numbers
•
A Random object performs complicated

calculations based on a seed value to produce a
stream of seemingly random values
•
See RandomNumbers.java (page 126)
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The Math Class
•
The Math class is part of the java.lang package
•
The Math class contains methods that perform
various mathematical functions
•
These include:

absolute value

square root

exponentiation

trigonometric functions
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The Math Class
•
The methods of the Math class are static methods
(also called class methods)
•
Static methods can be invoked through the class
name – no object of the Math class is needed
value = Math.cos(90) + Math.sqrt(delta);

•
See Quadratic.java (page 129)
•
We discuss static methods further in Chapter 6
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Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2004 Pearson Addison-Wesley. All rights reserved 3-25
Formatting Output
•
It is often necessary to format values in certain
ways so that they can be presented properly
•
The Java standard class library contains classes
that provide formatting capabilities
•
The NumberFormat class allows you to format
values as currency or percentages
•
The DecimalFormat class allows you to format
values based on a pattern
•
Both are part of the java.text package